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Re: [TowerTalk] Motor Driven vac used in 40M DE?

To: Jim Thomson <>
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?
From: "Richard (Rick) Karlquist" <>
Date: Tue, 29 Dec 2009 07:46:46 -0800
List-post: <">>

Jim Thomson wrote:
> Has anyone tried this b4 ?    I have a F-12 340N, used, of recent vintage.  
> OK... here's the deal.     I'm seriously thinking of installing a  Jennings  
> ceramic  10-1050 pf  @  15 kv  motor driven vac cap in series with the hot 
> leg of the DE. 
> On paper, after doing the math's... this should work... and result in a flat 
> swr across  the entire band.    The stock swr curve is ok.... but I want to 
> be able to operate at the opposite ends of the band.   F-12  measures the swr 
> curve  in the shack, using the FT-1000MP's  internal  SWR meter.... and  400' 
> of  LMR-400  to the yagi.     The real  swr curve, measured at the DE, is of 
> course, higher.  [worse]    
> Have any of you tried this b4.. or seen it done ?    The idea was to tune the 
> DE to the bottom of the band... or even a little lower.   Then  with 1050 pf 
> dialed in.. [ 21.7 ohms Xc]..... swr should be flat  at  7003 khz.    As I 
> move up the band, less pf required. .. [ more XC] 

I did some antenna range tests with my MonstIR, varying the driven 
element length while leaving the director and reflector fixed.
You can improve the SWR bandwidth doing this, but the gain and
F/B drop off as you get away from the optimum frequency
to which the parasitic elements are tuned.  You really need to
tune all 3 elements as is done in the MonstIR.

Rick N6RK

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